1. Field of the Invention
The present invention relates to an optical tap module which is used to monitor an optical signal in an optical fiber transmission line in a field of an optical communication or optical measurement.
2. Related Art
In an optical communication system or optical measurement system, in view of monitoring the system, the detection of presence/absence of signal light propagating through a transmission line, such as an optical fiber or the like, or its intensity is required. Further, when a device having an action, such as amplification or modulation, on signal light is used, signal light is monitored and the device gets the feedback action result. Such a monitoring function is basically implemented by branching off some of signal light from the transmission line through a branching unit having a predetermined branching ratio and by causing an optical detector to detect branched signal light.
As a unit for branching some of propagating light, for example, if light propagates through an optical fiber, a method is used in which the optical fiber is bent with a small radius of curvature, and some of light propagating through a core is extracted as leakage light, for example, by using a cut groove provided in a clad or the like. Further, if an optical waveguide includes an optical fiber, a method which uses a branched waveguide branched from the waveguide at a predetermined branching intensity ratio is used.
On the other hand, if light propagates through a space, a method is used in which an optical filter is used so as to transmit and extract propagating light. When the optical filter is used, by using a neutral density (ND) filter not having wavelength dependency as a filter, light can be extracted from signal light in a predetermined ratio. Besides, the ratio is constant, not depending on the wavelength. Such a branching unit is called an optical tap, and optical components constituting the optical tap are incorporated in a signal body, which is called an optical tap module. Further, an optical filter used in the optical tap is called a tap filter.
In an optical monitor (optical tap) disclosed in Japanese Patent Publication No. JP 2004-62144A, an input optical fiber and an output optical fiber face a tap filter such that their optical axes have a predetermined angle with respect to the tap filter, and a photodiode is disposed at the back of the tap filter as an optical detector. Light incident on the input optical fiber is reflected, for example, 95 to 99% by the tap filter and is coupled to the output optical fiber. The remaining 1 to 5% transmits the tap filter and reaches the photodiode. Therefore, optical intensity can be monitored, without causing optical signal intensity of propagating light to be significantly lowered.
In Japanese Patent Publication No. S62-269909A, a tap is disclosed in which two gradient index rod lenses face each other, and a tap filter is interposed therebetween. FIG. 1 shows a generalized optical system of the tap. This is a general optical system having a two-core optical fiber collimator, which uses a gradient index rod lens having a lens length of ¼ pitch (meandering cycle), and a focusing lens for converging light beams to the photodiode (in JP S62-269909A, a gradient index rod lens is used for the focusing lens).
Light propagating through the input optical fiber 101 is emitted from the end surface of the optical fiber as divergent light, and is converted into parallel light by the rod lens 102 to be incident on the tap filter 105. The tap filter 105 reflects most light (normally, 95 to 99%). Reflected light is converged by the rod lens 102, is coupled to the output optical fiber 103, and then returns to the transmission line. Light transmitted the tap filter 105 is converged by the focusing lens 104, and is incident on the photodiode 106 (hereinafter, the same parts are represented by the same reference numerals, and the descriptions thereof will be omitted).
One of important characteristics of such a tap module is unidirectional characteristic. The unidirectional characteristic means that only light incident from a predetermined direction is detected, and also indicates how much backward light from other directions can be removed. In recent years, the importance of the unidirectional characteristic is increasing accompanied by the spread of an optical amplifier and low cost. As an index representing unidirectional characteristic, directivity is used. The directivity is represented by a ratio of a current output Ipd1 of the photodiode when light is input from the input optical fiber and a current output Idp2 of the photodiode when light having the same intensity is input to the output optical fiber, and is defined by the following expression. As the directivity is increased, excellent unidirectional characteristic is obtained.Directivity (dB)=10×log (Ipd1/Ipd2)
In an optical tap module having the unidirectional characteristic, light (signal light) incident from the input optical fiber transmits the tap filter to be coupled to the photodiode. On the other hand, light (backward light) propagating backward from the output optical fiber is not transmitted to the photodiode.
This is useful to monitor the intensity of incident light at the previous stage of the optical fiber amplifier. An erbium-doped optical fiber amplifier obtains a large gain with respect to signal light. In this case, however, amplified spontaneous emission (ASE) is emitted from the amplifier to an input side, as well as signal light. If a normal optical tap is used at the previous stage of the optical amplifier, since ASE is incident from an output-side of the optical tap, a background level of the photodiode output rises, and thus it is difficult to accurately measure the intensity of incident light.
As a method of improving the unidirectional characteristic, in general, the following method is used. As shown in FIG. 2A, it is assumed that, in a two-core optical fiber collimator 110 having the two parallel optical fibers 101 and 103, and the gradient index rod lens 102, the arrangement distance of the input optical fiber 101 and the output optical fiber 103 is d. Further, it is assumed that a distance between a focal point of a light beam 107 (indicated by a solid line), which transmits the focusing lens 104 and then is incident from the input optical fiber, and a focal point of a light beam 108 (indicated by a dashed line) (hereinafter, referred to as “light from the output optical fiber”) when light is incident on the output optical fiber in the same direction as the input optical fiber) is D.
D can be adjusted by suitably selecting the focal distance of the focusing lens and the optical fiber distance d. By widening the distance D, as shown in FIG. 2B, if the light beam 107 from the input optical fiber 101 is aligned so as to be converged to an active area of the photodiode 106, even when backward light is incident from the output optical fiber 103, light can be converged outside the active area.
Table 1 shows examples of the design values when the gradient index rod lens is used as the focusing lens. Herein, a lens A has a lens length of 0.15 pitch, and a focal distance of 2.41 mm, and a lens B has a lens length of ¼ (0.25) pitch and a focal distance of 1.95 mm.
TABLE 1OPTICAL FIBERFOCAL POINT DISTANCE D [μm]DISTANCE d [μm]LENS ALENS B125160125165200165200250200300370300
As described above, it can be understood that D can be made larger by using the gradient index rod lens having a short pitch. Actually, by incorporating an optical system having D larger than 120 μm and a general pin photodiode having a diameter of the active area of about 80 μm, the directivity of about 20 to 25 dB can be obtained.
However, in the related art, there are the following problems.
The directivity of the optical tap module required for the optical fiber amplifier is about 30 dB, and thus insufficient directivity is obtained in the configuration according to the related art.
Due to the following reasons, the directivity should be sufficiently made large. In general, the photodiode 106 is mounted on a nail pin 112 which serves as a chip mounting support having a wide front end, as shown in FIG. 2B (the diameter of the wide portion is 0.86 mm to the maximum). Assuming that the active area of the photodiode is designed to be at the center of the chip, if the size of a chip of the photodiode 106 is smaller than two times as much as the above-described distance D between the focal points of light, the focal point of light from the output optical fiber 103 is not located on the photodiode chip. However, if the wider portion of the nail pin 112 extends, the light beam 108 from the output optical fiber 103 may be irradiated onto the nail pin 112. Light irradiated onto and reflected and scattered by the nail pin may be reflected in a package 109 of the photodiode 106 and may be coupled to the photodiode 106 again as noise. As a result, the directivity is lowered.
When the incident light beam 107 from the input optical fiber 101 is irradiated onto the center of the active area of the photodiode 106, and the size of the chip of the photodiode 106 is larger than two times as much as the distance D between the focal points, the light beam 108 from the output optical fiber 103 is irradiated onto the chip, not necessarily onto the active area of the chip.
When a light-shielding film is provided in the chip, light irradiated onto the chip is reflected by the light-shielding film and becomes stray light. When the light-shielding film is not provided, light is absorbed outside the active area of the chip. However, some of generated electric carriers reaches the active area by diffusion and becomes noise. As a result, the directivity may be lowered.
In order to solve the above-described problems, the focal point distance D may be set large such that light from the output optical fiber 103 is not irradiated onto the nail pin 112 (FIG. 3). However, if doing so, the photodiode 106 should be located at a position distant from a center axis 120 of the lens. In general, when a lens having a circular circumference and the photodiode 106 are assembled as a module, preferably, the photodiode 106 is housed in a cylindrical container (package) 109, and the entire module is housed in a cylindrical case. However, when the photodiode 106 should be located at the position distant from the center axis 120 of the lens, the nail pin 112 is initially provided at the center of the package 109, and thus misalignment between the center of the package 109 and the center axis 120 of the lens occurs as shown in FIG. 3. In such a case, the alignment is difficult, and the entire module is rarely housed in the cylindrical case.
Further, when two rod lenses having ¼ pitch are used, a focus is formed on an end surface on the output side of a focusing rod lens. A distance needs to be provided between a focal surface and the photodiode, and light, which forms the focus on the lens end surface once, is widened again by that distance. For this reason, a photodiode having a large active area of photodetector needs to be provided. Accordingly, reflection easily occurs at the surface of the chip, and thus the directivity deteriorates.
In general, a component for optical communication which has small reflection feedback light is required, and the optical tap is no exception. In the optical tap, there is a problem in that light incident from the input optical fiber is reflected at the surface of each component and returns toward a light source.